1. Field of the Invention
The present invention pertains to offshore drilling units. In particular, it concerns offshore drilling units of the self-elevating type having a floatable hull and a plurality of legs movable from raised positions, in which the legs are supported by the hull in a body of water, to lowered positions in which the hull is supported by the legs on the floor of the body of water. More specifically, the present invention pertains to unique and improved construction for the legs of such a drilling unit.
2. Description of the Prior Art
There are several different types of rigs by which marine drilling is conducted. One of the first developed was the fixed platform rig in which the legs or supports of the rig are permanently installed, penetrating the floor of the body of water in which the well is to be drilled. Such a rig is limited by water depth and does not provide the mobility and flexibility of the mobile or portable type drilling rigs. For very deep water drilling, floating platforms or drill ships may be used. Depending upon the situation, drilling operations may also be conducted from submersible, semi-submersible, or self-elevating platform drilling units.
For use in water depths up to three hundred feet, and lately up to six hundred feet, the self-elevating platform or "jack-up" rig is quite popular. Such a rig is usually provided with a plurality of legs, frequently three or four in number, which are lowered from a floating platform or hull into the water for penetration of the water body floor. The platform is then elevated on the legs a sufficient distance above the water surface to prevent the platform from being subjected to wave action. Such rigs offer the advantage of being highly mobile, yet very stable when in place.
Some of the major design considerations in a self-elevating drilling rig are the construction of the legs and the means for elevating the platform or hull on the legs. Since the legs support the entire unit when in place, they must be designed for a great amount of strength and minimal effect from the constantly changing wave conditions. Three or four legged units, in which the legs comprise mutually parallel chord members interconnected by structural bracing members are popularly employed. The bottom of the legs are provided with spud feet or spud tanks for footing penetration of the floor of the body of water in which the unit is to be used. Examples of such units may be seen in U.S. Pat. Nos. 3,044,269 and 3,606,251.
One method of moving the legs between raised and lowered positions is to provide suitable rack and pinion arrangements. The pinions, which may be driven by powered elevating or jack units on the hull of the ship, are engageable with racks attached along the chord member of the legs. Rotation of the pinions causes the racks, and consequently the legs, to move in a vertical direction within wells provided in the hull. Such elevating mechanisms are shown in the above-mentioned U.S. Pat. Nos. 3,044,269 and 3,606,251. Others are shown in U.S. Pat. Nos. 2,924,077 and 3,014,346.
In the past, the legs of most jack-up drilling units have been constructed either in cylindrical form or as a "space frame" made up of angles, "I" beams or prismoidal structural of trapezium of tubular section. More recently, the legs have been constructed predominantly of tubular structural members. In any case, the vertical members or "chords", as they are sometimes referred to, are usually rigidly interconnected by horizontal and inclined or diagonal bracing members which are welded thereto. It is very difficult to obtain good welds under such circumstances and very often stress concentration points are created at points of the welds. Past experience has led to cracking and sometimes failure at such connections particularly in areas where severe loadings occur, often involving fatigue effects. This is particularly true in tubular construction, since the tubular members must be cut in such a fashion as to conform closely with curvilinear surfaces of another tubular member or quadrilateral sectioned member at inclined angles thereto.
The welding process causes imputs of heat to the structural sections which can easily cause distortion. Such "out of straightness" has frequently been a problem in jackup leg construction in prior art units, particularly when assymmetric leg chord sections are used. The symmetry and construction used have greatly reduced such problems.
In an effort to overcome some of the welding and stress concentration problems involved in connecting horizontal and diagonal bracing to the chord members of legs, preformed joints have been developed. An example of developments in this may be seen in U.S. Pat. Nos. 3,596,950 and 3,668,876.
Since the legs support the entire weight of the rig, when in position for drilling, they are required to be designed for great strength. When supported on the floor of a body of water, the greatest bending moments in the legs occur near the hull or platform. This means that the chord members must necessarily be stronger near the upper portion of the leg. In legs which use tubular chord members this requires that the wall thickness of the tubular members increase from the bottom of the leg upwardly. Non-uniform wall thickness of the chord member creates problems in construction of legs. For example, welds are harder to make and points of stress concentration are created. If the tubular chord members are not reduced in wall thickness toward the bottom of the legs, the unnecessary increased metal results in greater weight of the legs and increased costs thereof. Greater weights are particularly undesirable when the legs are in the raised, in-transit position, since the center of gravity of the unit is raised making the rig less stable in the water.
When a rack is welded to chord members, additional forces of the drive pinion requires additional strength in the chord member. Although the welding of longitudinal racks to the chords of a leg may increase the strength thereof, most jack-up rig legs depend upon the wall thickness of the chord member and the interconnecting bracing for strength.
Development continues in offshore drilling rigs in an attempt to build more economical, efficient and trouble free units. Since drilling is taking place in deeper and deeper water, floating rigs and self-elevating platform rigs are in the forefront of this development.